As methods for smelting nickel oxide ore which may also be called limonite or saprolite, known are a dry smelting method for producing nickel matt using a flash smelting furnace, a dry smelting method for producing an iron-nickel alloy (ferronickel) using a rotary kiln or moving hearth furnace, a wet smelting method for producing mixed sulfide using an autoclave and the like.
Dry smelting of saprolite ore commonly includes roasting the ore in a rotary kiln, and then melting the roasted ore in an electric furnace to obtain a ferronickel metal, and then separating a slag. At this time, some iron is allowed to remain in the slag for maintaining the concentration of nickel in the ferronickel metal at a high level. However, it disadvantageously requires a large amount of electric energy because the whole amount of saprolite ore needs to be melted to generate a slag and a ferronickel.
Patent Document 1 discloses a method including inputting oxidized nickel ore and a reducing agent (anthracite) into a rotary kiln, and reducing the ore in a semi-molten state to reduce parts of nickel and iron into metal, and then recovering a ferronickel by gravity separation or magnetic separation. Advantageously, according to the above method, a ferronickel metal can be obtained without performing electric melting, leading to reduced energy consumption. However, the method suffers from the following problems: reduction is performed in a semi-molten state, and thus the produced metal will be dispersed in the form of small particles; and the yield of nickel metal will be relatively low partly due to losses during gravity separation and magnetic separation.
Further, Patent Document 2 discloses a method for producing a ferronickel using a moving hearth furnace. The method described in the above document includes mixing raw materials containing nickel oxide and iron oxide with a carbonaceous reducing agent to form a pellet, and heat-reducing the mixture in a moving hearth furnace to obtain a reduced mixture, and then melting the reduced mixture in a separate furnace to obtain a ferronickel. The document describes that alternatively, both slag and metal or one of either may be melted in a moving hearth furnace. However, melting the reduced mixture in a separate furnace requires a large amount of energy as in the melting process in an electric furnace. Further, disadvantageously, the slag and the metal may be fused to the furnace floor when melted in the furnace, resulting in difficult discharge from the furnace.
Here, with regard to the nickel grades in iron-nickel alloys, the Japanese Industrial Standard (JIS) specifies the nickel grade in a ferronickel as shown in Table 1. According to this, the nickel grade in a ferronickel needs to be 16% or more for a commercial ferronickel.
TABLE 1Chemical components (%)TypeAbbreviationNiCSiMnPSCrCuCoHighNo. 1FNi H116.0 or3.0 or3.0 or0.3 or0.05 or0.03 or2.0 or0.10 orNi ×carbonmoremorelesslesslesslesslessless0.05 orferronickellessNo. 2FNi H216.0 orless than5.0 or0.3 or0.05 or0.03 or2.5 or0.10 orNi ×more3.0lesslesslesslesslessless0.05 orlessLowNo. 1FNi L128.0 or0.02 or0.3 or—0.02 or0.03 or0.3 or0.10 orNi ×carbonmorelesslesslesslesslessless0.05 orferronickellessNo. 2FNi L217.0 or0.02 or0.3 or—0.02 or0.03 or0.3 or0.08 orNi ×more andlesslesslesslesslessless0.05 orless thanless28.0
Patent Document 1: Japanese Examined Patent Application Publication No. H01-021855
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2004-156140